1. Field of the Invention
The present invention relates to an electrode binder which is enhanced in its adhesion and high capacity characteristics. The present invention further relates to a method for manufacturing an active material slurry for a lithium-ion secondary battery, which contains the electrode binder.
2. Description of the Prior Art
Generally, a lithium-ion secondary battery is a rechargeable battery which is widely used in many applications.
The lithium-ion secondary battery uses a lithium transition metal oxide as a positive electrode and a carbon or graphite as a negative electrode. As an electrolyte, a compound solvent consisting of cyclic carbonates and linear carbonates, to which lithium hexafluoro phosphate (LiPF.sub.6) or lithium tetrafluoro boronate (LiBF.sub.4) as a supporting electrolyte is added, is used.
Accordingly, when the lithium-ion secondary battery is charged, lithium ions deintercalated from the positive electrode are inserted to the carbon or graphite of the negative electrode. During discharge of the lithium-ion secondary battery, lithium ions deintercalated from the carbon or graphite are inserted to the lithium transition metal oxide of the positive electrode.
The positive and negative electrodes applied with the active material are obtained by passing positive and negative substrates through a container containing an active material slurry, and drying and pressing the same.
The active material slurry consists of active material such as lithium transition metal oxide as the positive electrode and carbon or graphite as the negative electrode, a conducting agent and binder.
Polyvinylidene fluoride (PVDF) which is a fluoride polymer and is less reactive with the organic solvent which is used as the electrolyte is usually used as the binder. Alternatively, polyimide (PI) and rubber which are the polymer materials are also used as the binder.
The performance of the battery mostly depends on kinds of the active material used, and on the kinds of the binder contained in the active material slurry. For example, the cycle life, high capacity, and low temperature characteristic of a battery depend on the types of polymer of PVDF.
That is, if homopolymer of PVDF is used as the binder, required adhesion can be obtained, but it is difficult to coat the active material on the electrode substrate at a uniform thickness due to low liquidity of the active material slurry.
If copolymer of PVDF is used, it is easy to coat the active material on the electrode substrate and the high capacity of the battery can be obtained. However, since the electrolyte content rate becomes higher, the volume expansion of the battery is increased. This results in the active material being separated from the substrate.